How to check for unrestricted Internet access? (captive portal detection)

Question

I need to reliably detect if a device has full internet access, i.e. that the user is not confined to a captive portal (also called walled garden), i.e. a

Answer 1

if you are already using retrofit you can do it by retrofit. just make a ping.html page and send an head request to it using retrofit and make sure your http client is configured like below: (followRedirects(false) part is the most important part)

private OkHttpClient getCheckInternetOkHttpClient() {
    return new OkHttpClient.Builder()
            .readTimeout(2L, TimeUnit.SECONDS)
            .connectTimeout(2L, TimeUnit.SECONDS)
            .followRedirects(false)
            .build();
}

then build your retrofit like below:

private InternetCheckApi getCheckInternetRetrofitApi() {
    return (new Retrofit.Builder())
            .baseUrl("[base url of your ping.html page]")             
            .addConverterFactory(GsonConverterFactory.create(new Gson()))
            .client(getCheckInternetOkHttpClient())
            .build().create(InternetCheckApi.class);
}

your InternetCheckApi.class would be like:

public interface InternetCheckApi {
    @Headers({"Content-Typel: application/json"})
    @HEAD("ping.html")
    Call checkInternetConnectivity();
}

then you can use it like below:

getCheckInternetOkHttpClient().checkInternetConnectivity().enqueue(new Callback() {
     public void onResponse(Call call, Response response) {
       if(response.code() == 200) {
        //internet is available
       } else {
         //internet is not available
       }
     }

     public void onFailure(Call call, Throwable t) {
        //internet is not available
     }
  }
);

note that your internet check http client must be separate from your main http client.